Method for producing a thermoplastic sandwich composite structure

Abstract

A method is described for producing a sandwich composite structure which has at least one structured core layer, abbreviated SKS, made of thermoplastic material, which has two opposing core layer surfaces, each of which is directly or indirectly bonded to a thermoplastic cover layer.The invention is characterized by the following method steps: a) draping the SKS to obtain a structured core layer, vSKS for short, pre-formed in a predetermined 2- or 3-dimensional shape, b) forming one of the two thermoplastic cover layers on one of the two core layer surfaces of the vSKS as part of a spatially and dynamically guided and locally limited processing process along the core layer surface of the vSKS, in which the thermoplastic material of the core layer surface of the vSKS is locally heated and a band-shaped thermoplastic material is applied to the locally heated core layer surface of the vSKS, and c) forming the other of the two thermoplastic cover layers on the other of the two core layer surfaces of the vSKS using the processing process as under b).

Description

Technical area

[0001] The invention relates to a method for producing a sandwich composite structure which has at least one structured core layer, abbreviated to SKS, made of thermoplastic material, which has two opposing core layer surfaces which are each directly or indirectly bonded to a thermoplastic cover layer.

[0002] Thermoplastic sandwich composite structures have gained importance in recent years due to their advantages over sandwich composites made of thermosetting materials, including their recyclability, as well as other benefits. Sandwich structures offer an excellent, material-appropriate construction method for producing lightweight structures in small and large quantities for a wide variety of applications. State of the art

[0003] Sandwich composite structures are widely used in lightweight construction because they offer very high surface load characteristics combined with low specific density. For highly stressed structural components, these sandwich composites typically use structured core layers, for example, in the form of a honeycomb structure, bonded on both sides to a fiber-reinforced plastic cover layer.

[0004] In the production of 3-dimensionally shaped thermoplastic sandwich composite structures, flat, plate-shaped, preferably honeycomb-structured core layers made of thermoplastic material are initially used, which, with or without cover layers, which in turn are preferably made of fiber-reinforced thermoplastic material, are subjected to a shaping process in the form of semi-finished products, in which it is important to convert the core layer into a desired final shape while retaining its structure as much as possible.

[0005] The publication DE 10 2011 006 819 A1 discloses a method for producing a three-dimensionally contoured sandwich structure consisting of two thermoplastic cover layers and a core layer with a honeycomb structure located between them. For the purpose of three-dimensional contouring of the sandwich structure, the honeycomb-like core layer is pre-machined using a machining or separating process to achieve the desired three-dimensional contour. Subsequently, the thermoplastic cover layers are bonded to the core layer by means of a hot-pressing process.

[0006] EP 1 993 808 B1 discloses a method for producing a three-dimensional sandwich structure, which is transferred to a molding press with molding dies that can be deflected in different directions one after the other for shaping purposes. The core layer located inside the sandwich structure is compressed to varying degrees in different areas, leading to the formation of pleats.

[0007] EP 0 894 611 B1 describes a method for producing a component for motor vehicles by pressing a plate comprising at least a first and second cover layer and an intermediate, cellular core made of thermoplastic material, wherein both cover layers are formed of reinforced thermoplastic material. In areas of shape-related curvatures, additional material reservoirs are created, each of which counteracts any locally occurring reduction in thickness.

[0008] Publication WO 2020 / 200796 A2 describes a method for producing a sandwich composite component with a molded-in two- or three-dimensional shape, comprising at least one structured core layer made of thermoplastic material with two opposing core layer surfaces, each of which is directly or indirectly bonded to a thermoplastic cover layer. A flat sandwich semi-finished product, heated by infrared radiation, is preformed using a pressing tool and then final-formed. Between preforming and final forming, the preformed sandwich semi-finished product is stabilized by contact cooling.

[0009] However, the known manufacturing processes for forming sandwich composite structures are associated with limitations in the choice of shapes as well as restrictions with regard to the most optimal fiber flow in the final formed sandwich composite structure as well as with regard to its dimensional accuracy.

[0010] In particular, the trend in the aviation sector towards thermoplastic materials, especially the use of high-temperature thermoplastics, requires new processes for the production of complex geometries, since these can only be processed to a limited extent by thermoforming. Description of the invention

[0011] The object of the invention is to further develop a method for producing a sandwich composite structure which has at least one structured core layer made of thermoplastic material with two opposing core layer surfaces, which are each directly or indirectly bonded to a thermoplastic cover layer, in such a way that the realizable range of shapes for forming 3-dimensional, finally formed sandwich composite structures is to be expanded compared to previous possibilities. In particular, the use of high-temperature thermoplastics should be possible, since these can only be processed to a limited extent using conventional thermoforming processes. In addition, the production of fiber-reinforced sandwich composite structures with the best possible fiber flow within the structural regions should be made possible in order to improve the structural load-bearing capacity of such components while simultaneously reducing weight.

[0012] The solution to the problem underlying the invention is specified in claim 1. Features that advantageously further develop the inventive concept are the subject of the subclaims and the further description, in particular with reference to the explained embodiment.

[0013] According to the solution, the method for producing a sandwich composite structure having at least one structured core layer made of thermoplastic material and two opposing core layer surfaces, each of which is directly or indirectly bonded to a thermoplastic cover layer, is characterized by the following process steps: The starting product is a structured core layer made of thermoplastic material, typically in the form of a plate-shaped workpiece, which can be produced in almost any size in a manner known per se, for example by gluing tubes produced in an extrusion process with subsequent division or separation into plate-shaped piece goods. Such structured core layers are also commonly referred to as honeycombs. Of course, alternative processes are suitable for producing such structured core layers, e.g.generative manufacturing processes etc.

[0014] Depending on the selected thermoplastic material, the core layer thickness as well as the wall thickness of the honeycomb or cylindrical structures, the structured core layer present as the starting product can be formed in a rigid plate-like manner or in a flexible mat-like manner, but still retains its structure.

[0015] In a first process step, the structured core layer is transferred into a defined, predefined two- and preferably three-dimensional shape during a draping process to obtain a so-called preformed structured core layer. Depending on the nature of the structured core layer, which is available as a starting product or in the form of a semi-finished product, the shaping draping preferably takes place using a tool with a tool surface that determines the shape or final shape of the preformed structured core layer, to which the structured core layer is pressed flatly under the application of heat and / or pressure.

[0016] If the semi-finished product is a mat-like, flexible, formable, structured core layer with at least slight surface deformability, it can be deformed simply by placing it on the workpiece surface and, if necessary, by gently pressing it against the surface. The deformation process can also be supported by additional heat application, particularly in cases where the structured core layer is plate-shaped and tends to have greater surface stiffness at room temperature, so that the thermoplastic structured core layer.Heat is preferably applied using contactless infrared radiation until the thermoplastic softening temperature of the thermoplastic material of the structured core layer is reached, allowing it to be deformed while maintaining its structure and adapting to the three-dimensional shape defined by the workpiece surface through flat conformation. It is important to ensure that the heat application, which supports the draping process, acts gently on the structured core layer, so that after the shaping draping process, the structured core layer is deformed while maintaining its structure.

[0017] Preferably, the process of draping the structured core layer onto the forming tool surface is supported by a pressure application. The pressure application is preferably carried out using a stamp-like or roller-like tool, which is brought into contact with the freely accessible core layer surface under gentle and preferably flat pressure. The tool is preferably rolled over the entire freely accessible core layer surface by gently applying pressure or comes into contact with it by means of a stamp-like displacement process.

[0018] Alternatively, or in combination with the above-mentioned pressure application, a further preferred process variant provides for the application of negative pressure between the shape-defining tool surface and the structured core layer to be deformed, whereby the structured core layer is virtually sucked onto the entire surface of the shaping tool surface. In this case, it is advantageous if the structured core layer is covered on its surface with a thin thermoplastic film, preferably bonded by a material bond.

[0019] After completion of the shaping draping of the structured core layer to obtain a preformed structured core layer, a thermoplastic cover layer is formed on one of the two core layer surfaces of the preformed structured core layer by means of a spatially and dynamically guided and locally limited processing process along the core layer surface, in which the thermoplastic material of the core layer surface of the preformed structured core layer is locally heated and a band-shaped thermoplastic material is applied to the locally heated core layer surface of the preformed structured core layer to form a material bond.

[0020] In another variant of the molding process for obtaining the preformed, structured core layer, it is advisable to use a tool with a tool surface made of a transformer material, for example, a shape-memory material, which assumes a spatial shape imprinted on it when the temperature changes. The following forming process would therefore be possible: The structured core layer is first placed flat on the tool surface made of a shape-memory material and fixed there, e.g., by clamping, applying vacuum, etc. After activation of the transformer material, e.g., by changing the temperature, etc., the tool assumes the desired tool surface shape in its final form, which is automatically transferred to the core layer.

[0021] To form or create the thermoplastic cover layer on each of the two core layer surfaces of the preformed structured core layer, a tape layer is used, which is preferably mounted so that it can be freely positioned relative to the preformed structured core layer. Tape layers represent tool heads that are typically attached to a manipulator end of an industrial robot for their free positioning and locally deposit or print so-called tapes, preferably made of a fiber-reinforced thermoplastic material in the form of individual tape webs, on a substrate surface, in this case on the core layer surface.To form a large-area, fiber-reinforced thermoplastic cover layer, the tape is typically applied in parallel, adjacent webs that touch each other butt-to-end or partially overlap each other. The thermoplastic fiber-reinforced material applied to the core layer surface is heated above its melting temperature. A known tape layer is described, for example, in the publication DE 10 2007 009 124 A1.

[0022] Advantageously, a strip-shaped, fiber-reinforced thermoplastic material is used to form the thermoplastic cover layer to be formed on the preformed, structured core layer. The material has structure-reinforcing fiber components, preferably in the form of continuous fibers, which are arranged along the strip-shaped, thermoplastic material, which is available as a meter-long product. The strip-shaped thermoplastic material and the core layer surface onto which the application or printing process takes place are preferably heated locally in such a way that, during application, the strip-shaped thermoplastic material and the locally heated core layer surface bond together. The strip-shaped thermoplastic material preferably has a lower melting point than the thermoplastic material of the structured core layer, so that the application or printing processThe cover layer can be printed using the tape layer in a gentle and structure-preserving manner onto the preformed structured core layer, forming a material bond.

[0023] In particular, the heating of the core layer surface is carried out close to the surface with a low thermal depth effect, so that the thermoplastic material of the preformed structured core layer otherwise remains below the melting temperature, whereby no or only negligible structural deformations occur during the application of the cover layer.

[0024] Due to the limited thermal penetration into the core layer surface, the heated joining areas cool below the melting temperature immediately after the joining process, allowing the joining area to fully consolidate immediately after tape application, minimizing the need for further consolidation. Targeted heat application and in-situ consolidation also enable direct marginal sealing and the introduction of screw and fixation points through local, complete consolidation, for example, by increasing pressure or temperature.

[0025] In a preferred embodiment, the strip-shaped thermoplastic material and the core layer surface are locally heated, preferably contactlessly, with a common heat source immediately before applying the strip-shaped thermoplastic material to the core layer surface. The contactless heating is preferably carried out using an infrared radiation source, a gas burner, or a laser, which is attached to the tool head of the tape layer.

[0026] After the completion of the thermoplastic, fiber-reinforced cover layer on one of the two core layer surfaces of the preformed structured core layer, the other of the two thermoplastic cover layers is formed on the opposite core layer surface of the preformed structured core layer using the same machining process used to apply the first thermoplastic cover layer to the preformed structured core layer. However, this requires that the preformed structured core layer resting on the forming tool surface be removed from the tool to allow the tape layer free access to this core layer surface. Preferably, the component is already so rigid that no additional support mold is required.

[0027] The first cover layer, which is already applied to one side of the preformed structured core layer and is integrally bonded to it, is able to stabilize the preformed structured core layer in a shape-preserving manner and contributes to increasing its dimensional rigidity, so that the handling and further processing of the preformed structured core layer is at least facilitated.

[0028] To apply the further thermoplastic cover layer to the still unprocessed free core layer surface, the pre-formed structured core layer is only fixed at the edges, for example with a gripper system, and transferred to a position in which the still unprocessed core layer surface is freely accessible for the subsequent processing process.

[0029] Alternatively or in combination, it is possible to place or fix the preformed structured core layer, which is joined to a cover layer on one side, on another tool in a shape-preserving manner, e.g. by means of clamping, clamping or negative pressure, whereby the cover layer of the preformed structured core layer is oriented directly towards the tool.

[0030] In the same way in which the first cover layer is applied to the preformed core layer, the second cover layer is formed with the aid of a tape layerer, which is capable of applying strip-shaped thermoplastic material, which is preferably fiber-reinforced with continuous fibers, to the free core layer surface. The thermoplastic material webs, which are to be applied next to one another in web form, are applied with the aid of the tape layerer in a state heated above the melting temperature of the strip-shaped thermoplastic material, which is also heated above the melting temperature at least on the core layer surface, so that the fiber-reinforced strip-shaped thermoplastic material bonds firmly to the preformed, structured core layer.

[0031] The tape layer's free positioning allows the entire surface area of ​​the preformed, structured core layer to be bonded to the fiber-reinforced thermoplastic material. For this purpose, the tape layer is guided over the preformed, freely accessible core layer surface, with the fiber-reinforced, web-shaped thermoplastic material applied to the core layer surface being applied to the core layer surface using a predeterminable contact pressure and by specifying a specific fiber orientation or fiber orientation.

[0032] In a further preferred embodiment, prior to the application or deposition of the web-shaped thermoplastic material with the aid of the tape layer, a thermoplastic film is applied to the core layer surface of the preformed structured core layer to form a cover layer. The thermoplastic film preferably has a lower melting temperature than the melting temperature of the structured core layer. The thermoplastic film supports the structural retention of the structured core layer during the application of the cover layer with the aid of the tape layer and also supports the integral bond between the forming cover layer and the core layer surface to be provided with the cover layer.

[0033] The proposed method enables the production of precisely fitting sandwich structures with a high degree of design freedom and the possibility of integrating additional functions by freely draping a structured core layer to obtain a preformed structured core layer and subsequently covering the two core layer surfaces with a cover layer using a tape layer that can be spatially positioned anywhere and provides web-like, thermally softened thermoplastic material for the production of each cover layer. This process is particularly advantageous for components in small and medium quantities, which can also be reinforced or provided with additional local continuous fibers.

[0034] When draping the thermoplastic structured core layer, care must be taken not to compromise the core layer structure by shaping it into a desired three-dimensional shape. To support the draping process, the structured core layer must be heated above its thermoplastic softening temperature but kept well below its melting temperature. In this way, the structured core layer can be shaped in a defined manner, for example, using a forming tool with a forming tool surface.

[0035] In a sequential sequence, the two core layer surfaces of the preformed structured core layer are each completely covered or printed with a cover layer made of thermoplastic material during a tape-laying process. The tape layer used to form the cover layer provides a strip-shaped thermoplastic material heated above its melting temperature and applies it locally to the exposed core layer surface under pressure. To cover or print the entire core layer surface in a single layer with the web-shaped thermoplastic material, the tape layer is guided in a web-like manner over the entire core layer surface.This very approach to forming a cover layer by locally applying a thermoplastically softened material in sheet form enables the use of high-temperature thermoplastics, as heating is only required locally. Unlike other known processes involving global heating of the entire component, this approach has very narrow process windows or the desired shapes cannot be produced. Thus, the proposed process enables the shaping production of three-dimensional sandwich structures comparable to an additive manufacturing method through targeted temperature management, i.e., through a location-specific energy input tailored to the material selection for the cover layers and the core layer. Brief description of the invention

[0036] The invention is described below, without limiting the general inventive concept, using an exemplary embodiment with reference to the drawings. They show: Fig. 1 schematic representation of a preformed structured core layer with one-sided coating of a web-like thermoplastic material using a tape layer. Ways of implementing the invention, industrial applicability

[0037] Figure 1 shows a schematic representation of a preformed structured core layer 1, which has a honeycomb core layer structure and has already been converted into a three-dimensional form. In the illustrated case, the preformed structured core layer 1 is spatially fixed between two edge strips 2, 3, with the core layer surface 4 facing the plane of the drawing being positioned so that it is freely accessible.

[0038] A tape layer 6 is attached to the end of a spatially freely pivotable industrial robot 5, which is designed to apply a fiber-reinforced, band-shaped thermoplastic material 7 to the core layer surface 4. The tape layer 6 is capable of applying the band-shaped, fiber-reinforced thermoplastic material 7 to the core layer surface 4 by applying pressure and locally applying heat to the core layer surface 4. For this purpose, a heat source 8, preferably in the form of an IR radiation source 8, is additionally attached to the tape layer 6, which is capable of heating both the band-shaped, fiber-reinforced thermoplastic material 7 and edge regions of the thermoplastic core layer surface 4 in such a way that a materially bond is established between the band-shaped, fiber-reinforced thermoplastic material 7 and the core layer surface 4, which is locally heated above the melting temperature.However, the temperature input must be selected such that immediately after the formation of the adhesive bond, it consolidates, i.e., hardens, between the core layer surface and the cover layer 9. For this purpose, the heat input, the pressure application of the tape layer, and its movement speed must be suitably coordinated.

[0039] If necessary, the cover layer 9 can be formed in multiple layers by repeatedly passing over the applied band-shaped, fiber-reinforced thermoplastic material 7.

[0040] After completion of a top layer, it is necessary to Figure 1 illustrated preformed structured core layer is removed from the edge strips 2, 3 and the opposite free core layer surface 10 is covered with a typically full-surface continuous cover layer in the same way using the tape layer 6. List of reference symbols

[0041] 1 preformed structured core layer 2, 3 edge strips 4 core layer surface 5 industrial robot 6 tape layer 7 web-shaped, fiber-reinforced thermoplastic material 8 heat source 9 cover layer 10 core layer surface

Claims

1. A method for producing a sandwich composite structure comprising at least one structured core layer, abbreviated to SKS, made of thermoplastic material, which has two opposing core layer surfaces, each of which is directly or indirectly bonded to a thermoplastic cover layer, characterized bythe following process steps: a) draping the SKS to obtain a structured core layer preformed in a predetermined 2- or 3-dimensional shape, in short vSKS, b) forming one of the two thermoplastic cover layers on one of the two core layer surfaces of the vSKS by means of a spatially and dynamically guided and locally limited processing process along the core layer surface, in which the thermoplastic material of the core layer surface of the vSKS is locally heated and a band-shaped thermoplastic material is applied to the locally heated core layer surface of the vSKS to form a material bond, and c) forming the other of the two thermoplastic cover layers on the other of the two core layer surfaces of the vSKS using the processing process as under b).

2. Method according to claim 1, characterized in thatthe SKS has a honeycomb or cylinder-like structure that is retained when draped.

3. Method according to claim 1 or 2, characterized in that the strip-shaped thermoplastic material forming the cover layers has a lower melting point than the thermoplastic material of the SKS or corresponds to the thermoplastic material of the SKS.

4. Method according to one of claims 1 to 3, characterized in that structure-reinforcing fiber components are added to the band-shaped thermoplastic material.

5. Method according to claim 4, characterized in that the structure-reinforcing fiber components are formed as continuous fibers arranged along the band-shaped thermoplastic material.

6. Method according to one of claims 1 to 5, characterized in thatthe draping of the SKS is carried out using a tool with a tool surface that determines the shape of the vSKS, to which the SKS is pressed flatly under the application of heat and / or pressure.

7. Method according to claim 6, characterized in that by applying heat, the entire SKS is heated to or above a softening temperature k inherent to the thermoplastic material of the SKS, which, however, is below the melting temperature of the thermoplastic material of the KS.

8. Method according to claim 6 or 7, characterized in that the pressure is applied by means of a surface pressure that presses the SKS onto the tool surface on one side and / or the SKS is applied by means of a negative pressure applied between the tool surface and the SKS.

9. Method according to one of claims 1 to 8, characterized in thatthe formation of one of the thermoplastic cover layers on one of the two core layer surfaces of the vSKS is carried out by means of a tape layer which can be freely positioned spatially relative to the vSKS.

10. Method according to one of claims 1 to 9, characterized in that the band-shaped thermoplastic material is available as a metre-length product and is heated immediately before application to the locally heated core layer surface in such a way that the band-shaped thermoplastic material and the locally heated core layer surface bond together during application.

11. Method according to one of claims 1 to 10, characterized in that the strip-shaped thermoplastic material and the core layer surface are locally heated with a heat source immediately before applying the strip-shaped thermoplastic material to the core layer surface.

12. Method according to claim 11, characterized in thata non-contact heat source in the form of an IR radiation source, laser or gas heater is used as the heat source.

13. Method according to one of claims 1 to 12, characterized in that after the formation of one of the two thermoplastic cover layers, the vSKS connected to one thermoplastic cover layer is transferred into a position in which the other of the two core layer surfaces becomes freely accessible for the machining process.

14. Method according to claim 13, characterized in that the vSKS is placed and stored in the transferred position on the or another tool in a shape-retaining manner, with the cover layer of the vSKS facing the tool.

15. Method according to one of claims 1 to 14, characterized in that a thermoplastic film is applied directly to both core layer surfaces of the SKS, onto which the cover layer is subsequently applied.

16. Method according to claim 15, characterized in thatthe thermoplastic film has a melting temperature that is below the melting temperature of the SKS.

17. Method according to one of claims 6 to 16, characterized in that the structured core layer is placed flat on the tool surface made of a converter material and fixed there flat, and that the converter material is activated and the tool assumes a tool surface shape that corresponds to a final shape that is transferred to the core layer.

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